The invention relates to a process and a device for enriching water, in particular drinking water, with magnesium ions.
In order to protect installations such as pipelines, hot water generators and fittings from incrustation and corrosion caused by water which flows through them, the water is customarily treated. Since the incrustations are caused mostly by deposits of salts of hardness producing substances such as calcium and magnesium, softening equipment is usually employed to protect such installations. This equipment removes calcium and magnesium ions from the water. The function of the softening equipment is usually based on the ion exchange principle, whereby calcium and magnesium ions are either fully or partially replaced by sodium or potassium ions from an ion exchanger. In order to protect the installations from corrosion, corrosion protection agents can be added to the water in the form of polyphosphates and/or orthophosphates, silicates, carbonates and/or hydroxides, typically using a dosing system.
In addition to treatment of water performed in pipelines or other water supplying installations, other methods are used in which the water is treated directly at the tap location. This type of water processing operation carried out directly at the tap location, and sometimes also in separate containers, is referred to as processing at the point of use (“POU”). The systems used in POU processing are either arranged directly in front of or at the tap. Alternatively, open systems can be employed in which the water is processed in separate containers, typically cans.
Equipment for water processing operations is installed at the tap location directly at the outlet cock or just in front of it. For treatment of water in separate containers, it is known to use systems in which an insert or an attachment for a container is filled from above with the water to be processed, so that the water then flows through a fine filter in order to remove particles, and through activated carbon so as to remove chlorine, flavor imparting substances and odorous substances. The water then flows through an ion exchanger in a lower part of the container to remove hardness producing substances such as calcium and magnesium ions, heavy metals and hydrogen carbonate. These containers are known commercially as so-called “pitchers” or “jugs” and they are offered, for example, by the firms Anna and Brita.
The ion exchangers used in these containers contain mostly weakly acidic cation exchange resins in which the hardness producing substances and heavy metals present in drinking water are for the most part replaced with hydrogen ions from the ion exchanger. A consequence of this replacement is that the processed drinking water has a pH value in the approximate range of 4.5 to 5, while unprocessed drinking water or tap water mostly has a pH value of more than 7.
This lowering of the pH value by these commercial containers has consequences when the water processed in the container is used, as is often the case, not for drinking or cooking, but also for preparation of drinks using hot water, in particular, to make tea. Since only the pH value of the water, and not the ingredients of the tea, is responsible for the color of the tea, such tea becomes brighter and clearer as the pH value of the water is lowered. This is because tea leaves contain catechins, chlorophyll and flavonoids, which are natural pH indicators.
If the pH value is less than 4, the poured tea will be colorless. When the water has a pH of more than 7, the tea is darker, and oxidation of polyphenols which are contained in the tea leaves occurs at the same time. The polyphenols become crosslinked to polymers, which are insoluble in water and which form a thin film on the surface.
For this reason, weakly acidic cation exchange resins are employed in a buffered form in order to prevent lowering the pH value during ion exchange. In this buffered form they are conditioned or loaded to a certain extent with sodium ions or also with potassium ions, while the rest remains in the form of hydrogen ions. However, the total cation load of the raw water is not replaced during the exchange by hydrogen ions, since some of it will be replaced by sodium ions or potassium ions. The pH value of the processed water can be adjusted with this type of buffering to a value of more than 6.
However, it can be generally said that as a result of this treatment of water, physiologically important magnesium ions are removed, to a greater or lesser extent, from drinking water, which causes the quality of the drinking water to deteriorate. Moreover, the increase of the sodium content in drinking water is considered detrimental, in particular when the processed drinking water is used for preparations of meals for infants.
It is known in the art that calcium ions contained in drinking water can be exchanged for magnesium ions by means of ion exchangers with a strongly acidic exchange resin. In this context, a method is described, for example, in DE 100 20 437, wherein an ion exchanger is regenerated with a strongly acidic cation exchange resin, for example, by means of a solution of magnesium chloride. After the regeneration, the strongly acidic exchange resin of the ion exchanger is in the form of magnesium and can then release its magnesium ions in exchange for calcium ions during the preparation of drinking water. After the cationic ion exchange resin has been exhausted, the ion exchanger can again be regenerated with a magnesium chloride solution.
However, in contrast to a strongly acidic cationic ion exchange resin, a weakly acidic cationic exchange resin cannot be regenerated by means of a salt solution, such as, for example, with magnesium chloride. Weakly acidic cationic exchange resins which exist after an application for softening of water in the calcium form, namely, so that they are essentially loaded at 100% of their ion exchange capacity with calcium ions, are exhausted, and they can be regenerated only with acids. This is due mainly to the fact that weakly acidic cationic exchange resins contain as a rule carboxyl groups in the form of strong ions or exchange-active groups, onto which the calcium ions bond in the calcium form. The calcium ions are therefore only slightly dissociated in the ion exchanger and they are exchanged for the hydrogen ion of the acid. After the regeneration with an acid, the exchanger is again in the form of hydrogen ions, i.e., it is essentially loaded up to 100% of its ion exchange capacity with hydrogen ions.
Weakly acidic ion exchange resins can be conditioned after regeneration with an acid in a further preparatory stage, wherein they are converted, for example, with a sodium hydroxide solution or caustic potash solution into the sodium form or the potassium form, in which they are loaded with sodium ions or potassium ions instead of hydrogen ions.
It is also known from prior art that conditioned weakly acidic cationic exchange resins can be used in order to remove other cations, for example, ions of heavy metals or ions of hardness producing substances, from water. In this case, the heavy metal ions or ions of hardness producing substances are exchanged for sodium ions or potassium ions. However, if a weakly acidic cationic exchange resin is in the calcium form, an exchange of cations is no longer possible, with the exception of hydrogen ions.